Canonicalization

What is a canonical tag?

A canonical tag (aka “rel canonical”) is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or “duplicate” content appearing on multiple URLs. Practically speaking, the canonical tag tells search engines which version of a URL you want to appear in search results.

Code sample

Reasons to choose a canonical URL

There are a number of reasons why you would want to explicitly choose a canonical page in a set of duplicate or similar pages:

• To specify which URL that you want people to see in search results. 
• To consolidate link signals for similar or duplicate pages. It helps search engines to be able to consolidate the information they have for the individual URLs (such as links to them) into a single, preferred URL. This means that links from other sites to http://example.com/dresses/cocktail?gclid=ABCD get consolidated with links to https://www.example.com/dresses/green/greendress.html.
• To simplify tracking metrics for a single product or topic. With a variety of URLs, it’s more challenging to get consolidated metrics for a specific piece of content.
• To manage syndicated content. If you syndicate your content for publication on other domains, you want to ensure that your preferred URL appears in search results.
• To avoid spending crawling time on duplicate pages. You want Googlebot to get the most out of your site, so it’s better for it to spend time crawling new (or updated) pages on your site, rather than crawling the desktop and mobile versions of the same pages.

Specify a canonical page

To specify a canonical URL for duplicate URLs or similar pages, choose one of the following methods. Be sure to follow the general guidelines.

Canonical tag best practices

Duplicate content issues can be extremely tricky, but here are a few important things to consider when using the canonical tag:

1. Canonical tags can be self-referential

It’s ok if a canonical tag points to the current URL. In other words, if URLs X, Y, and Z are duplicates, and X is the canonical version, it’s ok to put the tag pointing to X on URL X. This may sound obvious, but it’s a common point of confusion.

2. Proactively canonicalize your home-page

Given that homepage duplicates are very common and that people may link to your homepage in many ways (which you can’t control), it’s usually a good idea to put a canonical tag on your homepage template to prevent unforeseen problems.

The moz.com homepage has a self-referential canonical tag.

3. Spot-check your dynamic canonical tags

Sometimes bad code causes a site to write a different canonical tag for every version of the URL (completely missing the entire point of the canonical tag). Make sure to spot-check your URLs, especially on e-commerce and CMS-driven sites.

4. Avoid mixed signals

Search engines may avoid a canonical tag or interpret it incorrectly if you send mixed signals. In other words, don’t canonicalize page A -–> page B and then page B -–> page A. Likewise, don’t canonicalize page A -–> page B and then 301 redirect page B -–> page A. It’s also generally not a good idea to chain canonical tags (A-–>B, B-–>C, C–->D), if you can avoid it. Send clear signals, or you force search engines to make bad choices.

5. Be careful canonicalizing near-duplicates

When most people think of canonicalization, they think of exact duplicates. It is possible to use the canonical tag on near-duplicates (pages with very similar content), but proceed with caution. There’s a lot of debate on this topic, but It’s generally ok to use canonical tags for very similar pages, such as a product page that only differs by currency, location, or some small product attribute. Keep in mind that the non-canonical versions of that page may not be eligible for ranking, and if the pages are too different, search engines may ignore the tag.

6. Canonicalize cross-domain duplicates

If you control both sites, you can use the canonical tag across domains. Let’s say you’re a publishing company that often publishes the same article across half a dozen sites. Using the canonical tag will focus your ranking power on just one site. Keep in mind that canonicalization will prevent the non-canonical sites from ranking, so make sure this use matches your business case.

Canonical tags vs. 301 redirects

One common SEO question is whether canonical tags pass link equity (PageRank, Authority, etc.) like 301 redirects. In most cases, they seem to, but this can be a dangerous question. Keep in mind that these two solutions create two very different results for search crawlers and site visitors.

If you 301 redirect Page A–>Page B, then human visitors will be taken to Page B automatically and never see Page A. If you rel-canonical Page A–>Page B, then search engines will know that Page B is canonical, but people will be able to visit both URLs. Make sure your solution matches the desired outcome.

How to audit your canonical tags for SEO

• View-source 

In most browsers, you can right-click to view-source, or simply type it into the address bar, like this: view-source:https://moz.com/learn/seo/cano… the source code, search for canonical tag in the <head>. If present, it should look like this:

• Use the MozBar
• Audit in bulk with software solutions